I also wondered this of Onix and Steelix. Both have height of around 30 feet. Would that refer to it's height or length? As that seems a bit short for length from head to tail.
If you look at a lampost (in the UK) they are 30 feet tall, and i imagine that's where onix/steelix's head to be (Probably because the anime). So are they measured in distance from the ground?? even though they are snakelike.